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When Urban Pollinator Gardens Meet Native Plant Communities

Public concern about the state of bees and other pollinating insects has led to increased interest in pollinator gardens. Planting a pollinator garden is often promoted as an excellent way for the average person to help protect pollinators. And it is! However, as with anything in life, there can be downsides.

In many urban areas, populations of native plants remain on undeveloped or abandoned land, in parks or reserves, or simply as part of the developed landscape. Urban areas may also share borders with natural areas, the edges of which are particularly prone to invasions by non-native plants. Due to human activity and habitat fragmentation, many native plant populations are now threatened. Urban areas are home to the last remaining populations of some of these plants.

Concern for native plant populations in and around urban areas prompted researchers at University of Pittsburgh to review some of the impacts that urban pollinator gardens may have and to develop a “roadmap for research” going forward. Their report was published earlier this year in New Phytologist.

Planting a wildflower seed mix is a simple way to establish a pollinator garden, and such mixes are sold commercially for this purpose. Governmental and non-governmental organizations also issue recommendations for wildflower, pollinator, or meadow seed mixes. With this in mind, the researchers selected 30 seed mixes “targeted for urban settings in the northeastern or mid-Atlantic USA” to determine what species are being recommended for or commonly planted in pollinator gardens in this region. They also developed a “species impact index” to assess “the likelihood a species would impact remnant wild urban plant populations.”

A total of 230 species were represented in the 30 seed mixes. The researchers selected the 45 most common species for evaluation. Most of these species (75%) have generalized pollination systems, suggesting that there is potential for sharing pollinators with remnant native plants. Two-thirds of the species had native ranges that overlapped with the targeted region; however, the remaining one-third originated from Europe or western North America. The native species all had “generalized pollination systems, strong dispersal and colonization ability, and broad environmental tolerances,” all traits that could have “high impacts” either directly or indirectly on remnant native plants. Other species were found to have either high dispersal ability but low chance of survival or low dispersal ability but high chance of survival.

This led the researchers to conclude that “the majority of planted wildflower species have a high potential to interact with native species via pollinators but also have the ability to disperse and survive outside of the garden.” Sharing pollinators is especially likely due to super-generalists like the honeybee, which “utilizes flowers from many habitat types.” Considering this, the researchers outlined “four pollinator-mediated interactions that can affect remnant native plants and their communities,” including how these interactions can be exacerbated when wildflower species escape gardens and invade remnant plant communities.

The first interaction involves the quantity of pollinator visits. The concern is that native plants may be “outcompeted for pollinators” due to the “dense, high-resource displays” of pollinator gardens. Whether pollinator visits will increase or decrease depends on many things, including the location of the gardens and their proximity to native plant communities. Pollinator sharing between the two has been observed; however, “the consequences of this for effective pollination of natives are not yet understood.”

The second interaction involves the quality of pollinator visits. Because pollinators are shared between native plant communities and pollinator gardens, there is a risk that the pollen from one species will be transferred to another species. High quantities of this “heterospecific pollen” can result in reduced seed production. “Low-quality pollination in terms of heterospecific pollen from wildflower plantings may be especially detrimental for wild remnant species.”

The third interaction involves gene flow between pollinator gardens and native plant communities. Pollen that is transferred from closely related species (or even individuals of the same species but from a different location) can have undesired consequences. In some cases, it can increase genetic variation and help address problems associated with inbreeding depression. In other cases, it can introduce traits that are detrimental to native plant populations, particularly traits that disrupt adaptations that are beneficial to surviving in urban environments, like seed dispersal and flowering time. Whether gene flow between the two groups will be positive or negative is difficult to predict, and “the likelihood of genetic extinction versus genetic rescue will depend on remnant population size, genetic diversity, and degree of urban adaptation relative to the planted wildflowers.”

The fourth interaction involves pathogen transmission via shared pollinators. “Both bacterial and viral pathogens can be transmitted via pollen, and bacterial pathogens can be passed from one pollinator to another.” In this way, pollinators can act as “hubs for pathogen exchange,” which is especially concerning when the diseases being transmitted are ones for which the native plants have not adapted defenses.

All of these interactions become more direct once wildflowers escape gardens and establish themselves among the native plants. And because the species in wildflower seed mixes are selected for their tolerance of urban conditions, “they may be particularly strong competitors with wild remnant populations,” outcompeting them for space and resources. On the other hand, the authors note that, depending on the species, they may also “provide biotic resistance to more noxious invaders.”

All of these interactions require further investigation. In their conclusion, the authors affirm, “While there is a clear potential for positive effects of urban wildflower plantings on remnant plant biodiversity, there is also a strong likelihood for unintended consequences.” They then suggest future research topics that will help us answer many of these questions. In the meantime, pollinator gardens should not be discouraged, but the plants (and their origins) should be carefully considered. One place to start is with wildflower seed mixes, which can be ‘fine-tuned’ so that they benefit our urban pollinators as well as our remnant native plants. Read more about plant selection for pollinators here.

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Believe it or not I’ve been grappling with this very issue for awhile when planting my garden as most of the seeds I plant are not a local ecotype. The reason being these seeds are just not available to buy. I even asked my local native plant society if this was ok and their answer was an extremely vague, “it depends.” So as gardeners must always do if they want to have a garden, I made my own decision based on my own observations. There are a few natural areas near my garden. One is a very small area about half a mile from my garden along a creek which is pretty much overrun with English ivy, honeysuckle and lesser celandine. The other area, a few miles away is the national park, Rock Creek Park, a fairly large forested area where large amounts of pesticides and resources are used to combat invasive species. The kinds of plants I plant naturally grow in sunny meadow conditions, not forested areas. I frequently visit these nearby natural areas and never see the plants I grow so I really can’t imagine the plants in my garden would spread and wipe out any rare native species in these areas. But of course this is only a guess. I think it would be very helpful for gardeners to know things like the specific locations of remnant native plant areas and how far does their garden need to be from these areas so as not to cause harm to these plants. That said, it’s GREAT that scientists are studying these issues and that you are making us aware of them.

You’re in luck. We have many excellent sources locally for local ecotype native plants and seeds. Archewilde Nursery, Earth Sanga Nursery, Herring Run Nursery (Blue Water Baltimore), Chesapeake Natives, to name a few. Use the online database: USFish and Wildlife Service Native Plants for the Chesapeake Region, if you live near Rock Creek Park, you are in the Piedmont and plants native to the Piedmont will do best. With locally sourced native plants and seeds we can hopefully reduce the detrimental effects until more research is available.

Thanks Ann for the excellent list of local plant nurseries! The thing about this report that really surprised me was it suggests “most plant species in six northeastern and mid-Atlantic USA cities are native (62–73%) and a fair proportion are ‘threatened’.” I knew that there were native plants, even rare native plants but I had no idea it was to such extent. If my city is one of the 6 cities this report is referring to that is. But if this is true than we probably should be doing what we can to conserve these remnant communities. I also have to say ecological gardening is getting more confusing by the minute. Corn poppy (Palaver rheas) a plant listed in the Invasive Plant Atlas, is at the bottom of this report’s list of plants most likely impact remnant native plants while dense blazing star, a plant often recommended for pollinator gardens is listed at the top of the report’s list as likely to impact native plants. So, I suppose buying native plants from nurseries that collect seed from local native plants probably is the safest way to go but also can get very expensive. I’ve also found many of the more weedy native plants grow on their own in my garden. But I get the feeling there isn’t a one size fits all solution and using a variety of strategies might be more effective. I like something Chris Helzer, scientist and author of the blog, The Prairie Ecologist said, “A plant’s status as native or not became less important than how it affected the diversity and function of the plant community it was part of.” If we can keep that in mind I think we might be on the right track.

Thank you very much for your insightful comments, Mara. I feel like we are in a similar place trying to wrap our heads around this issue. There is still so much we don’t know or understand. Hence, the need for MUCH more research. In the meantime, I think your comment about using a variety of strategies is the way to go. The important thing is that you are being observant and conscious about it – qualities that more of us should cultivate. Also, thanks for including the quote from The Prairie Ecologist blog. I’m pretty forgiving of most introduced species, so that sentiment really speaks to me.

The point of this article is valid, but the scenario rings untrue in a lot of landscapes. Our park “forests” in eastern North America can’t even be described as relicts of native communities, they are artifacts of human land manipulation. The ever-present Red Maple used to be a rarity….now it’s 40% of all woody stems…and invasive. And that goes for the parklands not already invaded by the mass-conscripted invasives of the 20th century (Kudzu, English Ivy, Japanese Honeysuckle, Barberry, Porcelainberry), and those are just the vines. And just a few of them. The scenario that somehow these parklands will be irreparably harmed by a few packets of asian poppies is kind of silly. BUT, again, the point is valid. Why not just transition the pollinator packets to native species?

Excellent point. I think Mara’s earlier comment echoes these concerns as she describes what the “natural areas” are like near her. (Check out her blog post as well: https://dirtysneaker.wordpress.com/2017/12/16/pollinator-garden-melt-down/) Disturbance is rampant in urban areas, so it makes one wonder what difference it really makes if we don’t plant precisely the right individual plants in our pollinator gardens and nothing more. And what are the “right” plants anyway? I think the real concern the authors of the paper have is with rare and threatened plants, and in the event that such plants are found in urban areas, how do we best protect them while also promoting healthy pollinator populations? In that case, at least for now, perhaps our best bet is to use as many locally sourced, native plants as we can. Other than that, as long as our gardens are, for the most part, free of noxious or invasive species, I think we are doing okay.